Urea is a leading source of solid form nitrogen in the fertilizer industry. This is because there are a number of advantages associated with urea over other fertilizers such as ammonium nitrate. In particular, urea has a high plant nutrient analysis. Urea is classified as a nonhazardous material whereas other fertilizers such as ammonium nitrate, under certain conditions, may be an explosive compound. Finally, environmental concerns such as minimizing air pollution are mitigated by using urea compared to many other fertilizers.
However, there are drawbacks associated with using urea as a fertilizer. When particulate urea is in the form of granules or prills, urea is highly friable due to relatively low hardness of the particles and prone to caking (prone to agglomeration). High friability is undesirable because the prills or granules tend to break into smaller particles, and produce substantial amounts of dust during handling, transportation, and application. Since urea products are often used in bulk blend fertilizer products, it is desirable for the urea to possess a predetermined, uniform size of granules to avoid unwanted segregation of the component products. If the urea product breaks into smaller particles, unwanted segregation of the urea may occur. Moreover, it is desirable for the urea to possess high hardness since urea granular products may break down when introduced into modern, rotating turbine-fan type field distribution equipment commonly used today. The breakdown of urea granules results in uneven distribution of the fertilizer from such equipment. And dusting is another undesirable consequence of relative low hardness.
Another concern with urea products is caking or agglomeration. Urea products are frequently transported, sold and used in large volumes and are thus transported and stored in bulk form. Because of the tendency for urea to cake or agglomerate, storage and transportation is not feasible without additional processing considerations that significantly add to the costs of production and/or handling. Furthermore, any additional processing must not involve undesirable reactions with the urea or raw materials used to make urea.
Attempts have been made to address these concerns. For example, it is known to incorporate formaldehyde and/or lignosulfonates into either the urea synthesis stream or the anhydrous molten urea melt stream. However, the use of formaldehyde is disfavored because it presents serious health and safety considerations. And the use of lignosulfonates can discolor the urea product to an unaesthetic brownish hue, with the result that the product is not well accepted in the marketplace.
The addition of a gelling type channelized 2:1 clay such as attapulgite or sepiolite to the urea melt stream or the urea synthesis stream is also known. Urea made with attapulgite or sepiolite has certain undesirable characteristics including color and increased agglomeration. Environmentally friendly urea products having increased hardness and increased resistance to agglomeration are desired.